The present invention is directed to the treatment of oil sludges, and in particular to biodegradation of oil sludges to environmentally-acceptable products. As such, the present invention is directed to the treatment of compositions with high sludge/total petroleum hydrocarbon concentrations, examples of which are oil refinery sludges, tank-bottom sludges from oil storage tanks or tankers, sludges from residues at oil wells, so called slop oil or treater emulsions, oil sludges from processing of solids coritaining oil wastes including centrifuged sludges, clay fines, and drilling mud residues. In contrast to waste water treatment processes utilizing low total petroleum hydrocarbon concentrations or processes for the production of single cell protein, biomass or bacterial cells.
Biodegradation of crude oil materials has primarily been directed to the clean up i.e bioremediation, of oil-contaminated soils and shorelines, as a result of onland oil spills from, for example, underground storage tanks, or from oil tankers at sea. Such bioremediation of hydrocarbons generally involves creation of conditions in the soil or on the shoreline that promote growth of microorganisms using the petroleum hydrocarbons, facilitating conversion of the hydrocarbons to biomass and/or their degradation, ultimately to carbon dioxide and water. The hydrocarbons are the source of carbon for microbial growth, although it may be necessary to add other ingredients, especially nitrogen and phosphorus, as fertilizers. Microorganisms also require a range of inorganic ions for growth, but such ions are generally present in adequate quantities in the soil that is being treated.
Bioremediation processes generally utilize aerobic microorganisms that require aeration/oxygenation by maximizing contact of the contaminated material with atmospheric oxygen through procedures of soil tilling or by aerating using positive or negative pressure air pumping systems.
The general hierarchy of microbial activity in crude oil is understood to be
aliphatics greater than  aromatics greater than  resins greater than  asphaltenes. Thus, aromatic and high molecular weight hydrocarbons are more difficult to degrade, compared to the lower alkanes.
Liquid-solid treatment systems have also been used to degrade petroleum hydrocarbons. However, long degradation treatment periods were encountered, e.g 50-100 days. Land treatment of waste crude oils and refinery oil sludges has been used for many years as a method of disposal of oil and sludge. Microbial growth and biodegradation rates tend to be suboptimal in land farming processes and the process is not easily controlled. In addition the process is influenced by soil composition, weather and temperature, as well as the methods used for tilling in the land farming process. For large refineries, large areas of land have to be committed to such a process, and moreover the first step in the process involves contamination of the soil with the oils to be degraded.
U.S. Pat. No. 3,899,376 discloses a single or multi-tank system that is primarily directed to waste water treatment. The process utilizes a particular bacterial strain from a culture collection for the bioremediation process.
U.S. Pat. No. 5,364,789 discloses a microbial cleaner comprising at least one hydrocarbon-ingesting microbe strain and a biocatalyst that transforms hydrocarbons into non-toxic substances. The biocatalyst includes a non-ionic surfactant, a chlorine-absorbing salt, at least one microbe nutrient and water. It is stated that the cleaner may be used in virtually any situation requiring the removal of hydrocarbons, including cleaning contaminated soil and treating oil spills on soil and water.
A method for the biodegradation of a petroleum hydrocarbon sludge fraction has now been found, such method using a reactor.
Accordingly, an aspect of the present invention provides a method for the biodegradation of an oil-based sludge, said oil-based sludge comprising a mixture of petroleum hydrocarbons, said method comprising the steps of:
(a) forming an aqueous mixture in a reactor of an oil-in-water emulsion, bacterial culture and nutrients for said bacterial culture,
said oil-in-water emulsion being an emulsion of said oil-based sludge in water,
said bacterial culture having the ability to grow on petroleum hydrocarbons as sole carbon source and having been isolated from a hydrocarbon contaminated soil or hydrocarbon-containing sludge or other environments rich in hydrocarbon degrading bacteria, by microbial enrichment techniques using hydrocarbons in the selection medium,
said reactor containing up to 50% by volume of total petroleum hydrocarbons;
(b) maintaining said aqueous mixture under aerobic conditions in the reactor at a temperature of at least 10xc2x0 C. for a period of time sufficient to reduce the amount of hydrocarbon by at least 25%, and at a pH conducive to the promotion of bacterial growth and hydrocarbon degradation; and
(c) discharging the aqueous mixture having a reduced amount of said hydrocarbons from said reactor.
In a preferred embodiment of the present invention, the nutrients comprise bioavailable nitrogen, phosphorous and potassium compounds, especially in which the nitrogen compound is an ammonium ion, nitrate or organic nitrogen, and the phosphorus is phosphate.
In another embodiment, the reactor contains about 5-50% by volume of total petroleum hydrocarbons, especially about 10-30% by volume of total petroleum hydrocarbons. The oil-based sludge preferably contains hexane-extractable hydrocarbons in an amount in the range of up to 500,000 ppm, especially in the range of 65,000-250,000 ppm. For clarity, the expression total petroleum hydrocarbons (or TPHs) as used herein is defined as hexane-extractable hydrocarbons including hexane soluble hydrocarbons.
In yet another embodiment, the nutrients are in proportions corresponding to relative proportions in bacterial cells, and supplied at concentrations which promote high levels of bacterial growth and high rates of hydrocarbon degradation.
In further embodiments, the petroleum hydrocarbons consist of mixtures of saturated hydrocarbons, aromatic hydrocarbons, hydrocarbon resins and asphaltenes, especially petroleum hydrocarbons obtained from petroleum refinery sludge, from the bottom of a storage tank for oil, from an on-land well head or from the washing of a hold in a tanker.
In other embodiments, the amount of nitrogen required to support the process is in the range of 50-1000 ppm, and preferably in the range of 300-700 ppm, and the minimum amount of phosphate is in the range of 10-200 ppm and preferably 50-150 ppm.
In additional embodiments, the aqueous mixture contains a surfactant, more especially a non-ionic or an anionic surfactant. The surfactant is in an amount sufficient to form said oil-in-water emulsion, especially in which the amount of surfactant is less than 2500 ppm and preferably less than 1500 ppm. It is preferred that the ratio of the amount of petroleum hydrocarbon to surfactant be at least 40:1.
The method of the present invention relates to the biodegradation of an oil-based sludge. The oil-based sludge comprises a mixture of petroleum hydrocarbons and may include non-petroleum solid or liquid contaminants and water. The petroleum hydrocarbon mixture would normally comprise a mixture of aliphatic hydrocarbons, aromatic hydrocarbons, hydrocarbon resins and asphaltenes.
The present invention is particularly directed to the biodegradation of a mixture of the petroleurm hydrocarbon from among the aliphatics, aromatics, resins and asphaltenes. Such mixtures of petroleum hydrocarbons may be obtained from a variety of sources. For instance, the mixture may be in form of a sludge obtained from a petroleum refinery. The sludge may also be obtained from the bottorm of a storage tank that has been used for the storage of petroleum oil, with the sludge being obtained particularly when the storage tank is cleaned or drained. Alternatively, the mixture of hydrocarbons could be a petroleum residue obtained from around an on-land well head, be an oil-containing clay fines material or be or from the cleaning of a hold of a tanker used for the transportation of petroleum products. The mixture of petroleum hydrocarbons, which is referred to herein as a sludge, may also be obtained from a variety of other sources. In each case, the sludge is characterized by having a substantial proportion of heavy end petroleum hydrocarbons which may require use of a solubilizing agent or surfactant to facilitate mixing and dispersal in water, as an oil-in-water emulsion.
The method of the present invention is carried out in a reactor. It is preferred that the reactor be a single stage reactor that is charged with the aqueous mixture described herein, allowed to incubate for a period of time to reduce the amount of hydrocarbons within the aqueous mixture, and then subsequently discharged from the reactor. Nonetheless, it is to be understood that the reactor could be in the form of a series of reactors in which the aqueous mixture is passed from reactor to reactor before being finally discharged from the process for the biodegradation of the sludge.
In the method, an aqueous mixture is fed to the reactor. The aqueous mixture is comprised of an oil-in-water emulsion, bacterial culture and nutrients for the bacterial culture. The sludge is in the form of the oil-in-water emulsion.
The amount of petroleum hydrocarbons fed to the reactor is primarily governed by the formation of the oil-in-water emulsion. In particular, the aqueous mixture may contain up to 50% by volume of total petroleum hydrocarbons. In preferred embodiments, the reactor contains 5-50% by volume of total petroleum hydrocarbons, especially 10-30% by volume.
The oil-based sludge contains hexane-extractable hydrocarbons. In preferred embodiments, the amount of hexane-extractable hydrocarbons is up to 500,000 ppm, especially in the range of 65,000-250,000 ppm.
It would normally be necessary to incorporate a surfactant into the aqueous mixture and to subject the aqueous mixture to agitation in order to form the oil-in-water emulsion. The surfactant is preferably a non-ionic or an anionic surfactant, and is used in an amount sufficient to form the emulsion. Nonetheless, the amount of the surfactant is preferably less than 2500 and particularly less than 1500 ppm. In addition, the amount of surfactant, if added, is maintained at as low a level as is consistent with obtaining the oil-in-water emulsion. In particular, it is preferred that the ratio of petroleum hydrocarbon to surfactant be at least 40:1, and especially at least 60:1.
The aqueous mixture also contains a bacterial culture. The bacterial culture used in the method of the present invention is a natural-occurring bacterial culture. Such a culture may be isolated from a hydrocarbon-contaminated soil or from hydrocarbons-containing sludge or from other environments, including soil or activated sludge, which may be rich in hydrocarbon-degrading bacteria, and inoculated in a basal medium, as described herein. The bacterial culture is selected by its ability to grow on petroleum hydrocarbons as the predominant source of carbon in the basal medium.
Bacterial enrichment techniques for isolation of a bacterial culture capable of growing on hydrocarbons are well understood in the art. Typical techniques comprise adding a sample of soil, sludge or other material containing a large population of bacteria to an aqueous medium containing hydrocarbons as the only or predominant carbon source. Other chemical components including an inorganic nitrogen source, phosphorous and salts necessary to support bacterial growth are also added. Such a medium can be used to preferentially promote multiplication of hydrocarbon-degrading bacteria using standard aerobic microbial cultivation methods, including incubation in aerated microbial culture vessels. By transfer of a small amount of the resultant growth culture to further samples of the same medium and repeating the process one or more times, an efficient hydrocarbon degrading culture is selected. The culture can be maintained or stored using methods well known in the art.
In order to prepare a high density culture for use as an inoculum for sludge degradation, the maintained culture may be inoculated into an aqueous medium consisting of the nutrients described herein, supplemented with petroleum hydrocarbons and incubated in an aerated reactor or fermenter or other culture vessel.
The preferred inoculum volume is 0.1-20% by volume of total culture volume, preferably 1-5% by volume. The preferred concentration of petroleum hydrocarbons used in this inoculum development medium is 0.5-5%, and can be obtained from various sources including petroleum sludges, crude oils or refined oils such as diesel oil.
A typical aeration rate of the inoculum reactor is 0.1-1.0 volumes of air per volume of medium per minute, with the culture incubated in the temperature range 20-37xc2x0 C. for 1-7 days, preferably at 27-33xc2x0 C., at a pH generally maintained in the range 6.5-8.0, preferably in the range 7-7.5. The resultant bacterial culture may be used to inoculate the reactor containing the sludge to be degraded, at a rate of 0.1-20% of total sludge volume, preferably 1-5%. Where a much larger volume of inoculum is required, the resultant inoculum may be transferred as an inoculum to a larger culture vessel and the culture development process repeated on the larger scale.
The aqueous mixture fed to the reactor also contains nutrients for the bacterial culture. A wide variety of nutrients for the bacterial culture may be used, as will be understood by persons skilled in the art. Such nutrients will include nitrogen, phosphorus and potassium compounds, and would normally also include a variety of other ingredients. In particular, the nutrients comprise bioavailable nitrogen and phosphorus compounds. In embodiments, the amount of nitrogen is in the range of 50-1000 ppm and preferably 400-700 ppm, and the amount of phosphate is in the range of 10-200 ppm and preferably 50-150 ppm. In addition to nitrogen and phosphorus compounds, the nutrient also contains optimized concentrations of compounds other than nitrogen, phosphorus, carbon, oxygen and sodium, required to support bacterial growth and therefore it is normally necessary to add to the reactor one or more of magnesium, manganese, inorganic or organic sulphur, calcium, iron, copper, cobalt, zinc, boron and molybdenum. It will be appreciated that a guide for selection of the relative amounts of nitrogen, phosphorus and other required nutrients is to relate their concentrations to the amounts of these components present in bacterial cells.
By providing an appropriate balance of nutrients and by adjustment of nutrient concentration, it is possible to achieve high levels of growth of hydrocarbon degrading bacteria and thus accelerated rates of hydrocarbon degradation. For example, Greasham (1993) xe2x80x9cBiotechnology, a multivolume comprehensive treatisexe2x80x9d (Eds, Rehm, H. J., et al) Vol. 3, p.131, VCH, Weinheim) has reported the typical non-carbon elemental composition of major bacterial components to be nitrogen 12.5%; phosphorus, 2.5%; potassium, 2.5%; sodium, 0.8%; sulphur, 0.6%; calcium, 0.6%; magnesium, 0.3%; copper, 0.02%; manganese, 0.01% and iron, 0.01%. Use of appropriate concentrations and ratios of nutrients tends to avoid a situation where growth is limited by depletion of one essential nutrient while all other nutrients may be present in excess.
The hydrocarbon provides the carbon source for growth; oxygen is obtained by aeration of the culture; sodium is provided in the form of caustic soda, required to adjust the pH. It is also understood that in some cases, some of these nutrient components may be present in sufficient quantities in some petroleum sludges or added water such that addition of selected nutrients may in some cases not be required. A disadvantage of relying on nutrients present as contaminants in sludge or water is that their concentrations may be variable, thus introducing inconsistencies into the process.
An example of a nutrient composition is as follows:
N (as NH4, NO3, or organic N): 500-700 ppm
P (as phosphate or related form): 100-120 ppm
K: 50-90 ppm
Mg: 10 ppm
Mn: 1-4 ppm
S (as sulphate or organic sulphur): 15 ppm
Ca: 8-12 ppm
Ferric Ion: 1 ppm
Copper: 0.5 ppm
Surfactant (nonionic or anionic): 1250 ppm
Co, Zn, B, Mo: 5-10 ppb each
The relative ratios of these nutrients are similar to the ratios typically found in the compositions of bacterial cells.
Other examples of nutrient compositions are given in the Examples herein below.
The aqueous mixture in the reactor is maintained at a temperature of a least 10xc2x0 C. Preferred temperatures are 15-37xc2x0 C., and especially 20-33xc2x0 C. The aqueous mixture is maintained in the reactor for a period of time sufficient to reduce the amount of total petroleum hydrocarbon by at least 25%, especially by at least 50%. Typical times to effect the reduction in total petroleum hydrocarbon is 5-20 days, depending on the petroleum hydrocarbon being treated and the reactor conditions.
Subsequent to maintaining the aqueous mixture at the predetermined temperature for a period of time, the aqueous mixture is discharged from the reactor. The aqueous mixture has a reduced amount of hydrocarbons, including a reduced amount of the hydrocarbons from the group comprising the aromatics, resins and asphaltenes.
The present invention may be used for the biodegradation of sludges, as described herein. In particular, it may be used for biodegradation of a combination of hydrocarbon components from among the fractions: saturates, aromatics, resins and asphaltenes.
It may also be used to preferentially degrade a proportion of the hydrocarbons, in a manner which causes the emulsion to break and facilitate separation of a water phase and a residual oil phase. The residual oil phase may be recovered for reuse. Alternatively, the oil phase may be recycled to the next reactor cycle with the water phase only being discharged from the reactor. The water phase contains high concentrations of hydrocarbon-degrading bacteria. Thus, the water phase may be used for processes including soil bioremediation processes, by direct spraying of the water on the contaminated soil. Alternatively, the bacteria may be recovered from the water phase by known methods (filtration or centrifugation) and subsequently the bacteria may be applied in these other processes.
Where subsequent batches of sludge are to be degraded in the reactor, a portion of the degraded sludge amounting for example, to 1-20% of reactor volume, may be retained in the reactor following discharge, as an inoculum source for the next sludge batch.
In addition to the above described batch sludge degradation process, it is envisaged that the invention extends to fed-batch, continuous and semi-continuous reactor processes. In the fed-batch process, after the batch process has proceeded for some time, additional sludge and/or nutrients/surfactant are added at one or more intervals and the process is allowed to continue. In continuous or semi-continuous processes, degraded sludge is removed from the reactor and replaced with undegraded sludge and nutrients/surfactants on a continuous basis or at intervals, respectively.